1. Field of the Invention
The invention relates to photodetectors for producing detectable signals from incident radiation, such as infrared radiation.
2. Description of the Related Art
Photodetectors made of mercury cadmium telluride (HgCdTe) semiconductor material are disclosed in U.S. Pat. No. 3,949,223, issued Apr. 6, 1976, and entitled, "Monolithic Photoconductive Detector Array."
As the '223 patent discloses, when radiation of the proper energy falls upon a semiconductor, its conductivity increases. Energy supplied to the semiconductor causes the covalent bonds to break, and electron/hole pairs (also called majority/minority carriers) in excess of those generated thermally are created. These increased current carriers increase the conductivity of the material. This so-called "photoconductive effect" in semiconductor materials is utilized in photodetectors.
Photodetectors, and particularly arrays of such photodetectors, have many applications. One application is the detection of infrared radiation. Infrared sensitive photodetector arrays are used for various heat and object sensing applications.
Typically, a photodetector array comprises a substrate that is electrically insulating, such as a dielectric or wide band-gap semiconductor, attached to a body of semiconductor material forming a region of first conductivity type (e.g., n-type) and a region of a second, opposite conductivity type (e.g., p-type) where the second region overlies the first region. Individual detectors of the array are delineated and isolated by masking, and then cutting or etching.
Present detector arrays have unacceptably high reflected light signatures which may compromise their effectiveness. Due to the imperfect absorption of the HgCdTe sensitive layer, for example, a significant amount of light is reflected from the areas of exposed metal (e.g., electrical contact pads), as well as non-planar surface features (e.g., mesa edges). It would be advantageous to minimize the undesirable reflections, thus reducing the optical signature of the detector, reducing optical cross-talk effects and reducing the risk of detectors generating ghost images.
While single detectors and detector arrays each possess optical signatures that may be reduced by minimizing undesirable reflections, optical cross-talk and ghost images are problems particular to detector arrays. Optical cross-talk and ghost images can be defined as the presence of any unwanted radiation impinging on any detector in an array other than the one onto which it is meant to be focused.